1. Field of the Invention
The present invention relates to a dual-mode band pass filter for use as a band filter, for example, in a communication apparatus used in a range of a microwave band to a millimeter-wave band.
2. Description of the Related Art
Conventional band pass filters used for high frequency bands include various kinds of dual-mode band pass filters (Miniature Dual Mode Microstrip Filters, J. A. Curtis and S. J. Fiedziuszko, 1991 IEEE MTT-S Digest, etc.).
Each of FIGS. 12 and 13 is a schematic plan view for illustrating a conventional dual-mode band pass filter.
In a band pass filter 200 shown in FIG. 12, a circular conductive film 201 is provided on a dielectric substrate (not shown). The conductive film 201 is coupled to input/output coupling circuits 202 and 203 arranged at an angle of 90 degrees relative to each other. In addition, a top-end open stub 204 is arranged so as to define an angle of 45 degrees with respect to the part where the input/output coupling circuit 203 is arranged. With this arrangement, since two resonance modes having different resonant frequencies are coupled to each other, the band pass filter 200 acts as a dual-mode band pass filter.
In addition, in a dual-mode band pass filter 210 shown in FIG. 13, a substantially square conductive film 211 is provided on a dielectric substrate. The conductive film 211 is coupled to input/output coupling circuits 212 and 213 disposed at an angle of 90 degree. Furthermore, a corner positioned at an angle of 135 degrees with respect to the input/output coupling circuit 213 is cut away to form a cut-away part 211a. With this arrangement, the resonant frequencies of two resonance modes are made different. As a result, since the two resonance modes are coupled to each other, the band pass filter 210 acts as a dual-mode band pass filter.
On the other hand, as an alternative to the circular conductive film 201 shown in FIG. 12, there is provided a dual-mode filter using a loop-shaped conductive film. That is, in each of Japanese Unexamined Patent Application Publication No. 9-139612 and Japanese Unexamined Patent Application Publication No. 9-162610, there is a dual-mode filter. This dual-mode filter incorporates a loop-shaped ring transmission line. In addition, as in the case of the dual-mode band pass filter shown in FIG. 12, input/output coupling circuits are arranged at a central angle of 90 degrees therebetween, and a top-end open stub is disposed at a part of the ring transmission line.
In the conventional dual-mode band pass filter shown in each of FIGS. 12 and 13, a two-stage band pass filter resonating at the two different resonant frequencies is provided. As a result, a miniaturized band pass filter can be obtained.
In each of the dual-mode band pass filters described above, however, the circular or square conductive film pattern has a structure that couples the input/output coupling circuits at each of the above specified angles, the coupling strength between the two resonance modes cannot be increased. Thus, there is a problem in that the pass band for the filter cannot be broadened.
In the band pass filter shown in FIG. 12, the conductive film 201 is circular. In the band pass filter shown in FIG. 13, the conductive film 211 is substantially square. That is, both conductive films 201 and 211 have limited configurations. As a result, in each of the above-described band pass filter, since the frequency band is determined by the dimensions of the circular or square conductive film, particularly, the position of an attenuation pole (the frequency) cannot be easily adjusted.
In order to overcome the problems described above, preferred embodiments of the present invention provide a method for adjusting the frequency of an attenuation pole of a dual-mode band pass filter. With this band pass filter, the above-described problems of the conventional art can be solved, and the size of the filter can be greatly reduced. In addition, the coupling strength between two resonance modes can be greatly increased. Furthermore, the dual-mode band pass filter of preferred embodiments of the present invention has a great deal of the freedom of design.
According to a first preferred embodiment of the present invention, a method for adjusting the frequency of an attenuation pole of a dual-mode band pass filter includes the steps of forming a metal film on a surface of a dielectric substrate or within the dielectric substrate, arranging a ground electrode such that the ground electrode overlaps with the metal film via at least a part of the dielectric substrate in a thickness direction of the dielectric substrate, forming at least one opening in the metal film to couple two resonance modes, coupling an input/output coupling circuit to the metal film, forming a coupling portion capacitively coupled to a perimeter of the metal film via a gap, and forming an input/output portion coupled to the coupling portion, and the input/output coupling circuit includes the coupling portion and the input/output portion. In this method, at least one of the coupling portion and the input/output portion is moved in a direction along the perimeter of the metal film.
According to a second preferred embodiment of the present invention, a method for adjusting the frequency of an attenuation pole of a dual-mode band pass filter includes the steps of forming a metal film on a surface of a dielectric substrate or within the dielectric substrate, arranging a ground electrode such that the ground electrode overlaps with the metal film via at least a part of the dielectric substrate in a thickness direction of the dielectric substrate, forming at least one opening in the metal film to couple two resonance modes, and forming an input/output coupling circuit coupled to the metal film. In this method, the input/output coupling circuit is defined by one of a strip line and a microstrip line. One end of the strip line or the microstrip line is directly and electrically connected to the metal film. A point for coupling the strip line or the microstrip line to the metal film is moved on the perimeter of the metal film.
According to a third preferred embodiment of the invention, a method for adjusting the frequency of an attenuation pole of a dual-mode band pass filter includes the steps of forming a metal film on a surface of a dielectric substrate or within the dielectric substrate, arranging a ground electrode such that the ground electrode overlaps with the metal film via at least a part of the dielectric substrate in a thickness direction of the dielectric substrate, forming at least one opening in the metal film to couple two resonance modes, and forming an input/output coupling circuit coupled to the metal film. In this method, the metal film and the input/output coupling circuit are located on different layers of the dielectric substrate. The input/output coupling circuit overlaps with the metal film via the dielectric layer so that the input/output coupling circuit is capacitively coupled to the metal film. A point for coupling the input/output coupling circuit to the metal film is moved along the perimeter of the metal film on the dielectric layer.
According to a fourth preferred embodiment of the present invention, a method for adjusting the frequency of an attenuation pole of a dual-mode band pass filter includes the steps of forming a metal film on a surface of a dielectric substrate or within the dielectric substrate, arranging a ground electrode such that the ground electrode overlaps with the metal film via at least a part of the dielectric substrate in a thickness direction of the dielectric substrate, forming at least one opening in the metal film to couple two resonance modes, forming an input/output coupling circuit coupled to the metal film, and forming an insulating layer having a via-hole electrode between the input/output coupling circuit and the metal film. In this method, one end of the via-hole electrode is electrically connected to the input/output coupling circuit and the other end thereof is electrically connected to the metal film. Positions for connecting the via-hole electrode to the input/output coupling circuit and the metal film are moved along the perimeter of the metal film.